Several publications and patent documents are cited throughout the specification in order to describe the state of the art to which this invention pertains. Each of these citations is incorporated herein by reference as though set forth in full.
Accumulation of soluble sugars is a characteristic trait in two closely related plant species, sorghum [Sorghum bicolor (L.) Moench] and sugarcane (Saccharum spp.) (1, 2). In both species, sucrose is the main type of sugar and accumulates in the parenchyma tissue of juicy stems. Sorghum belongs to the tribe of the Andropogoneae that includes potential biofuel crops like switchgrass, Miscanthus and successful biofuel crops like corn and sugarcane.
However, from a genomics point of view sorghum contains a simpler genome because it lacks the additional rounds of whole genome duplication events present in other species. Therefore, it has become possible to generate a high-quality genome sequence. Furthermore, cultivars exists that rival sugarcane in levels of stem sugar so that a genetic approach can be used to investigate which genes are differentially expressed to achieve high levels of stem sugar.
Small RNAs (18-25 nt) regulate many developmental and physiological processes in plants through the regulation of gene expression at either the transcriptional or post-transcriptional level (Chuck G, et al., (2009) Current Opinion in Plant Biology, 12:81-86; Vaucheret H. (2006) Genes Dev 2006, 20:759-771; Zamore P D, Haley B. (2005) Science, 309:1519-1524). They can be subdivided into short-interfering RNAs (siRNAs) and microRNAs (miRNAs) (Bartel D P. (2004) Cel, 116:281-297; Vazquez F. (2006) Trends in Plant Science, 11:460-468).
MicroRNAs are derived from capped and polyadenylated primary (pri)-miRNA transcripts that are transcribed by RNA polymerase II and can form a hairpin-loop structure by intramolecular pairing. Two sequential cleavages mediated by DICER LIKE 1 (DCL1) are required to produce a mature miRNA. In the first cleavage, DCL1 cleaves near the base of the hairpin-loop stem of the pri-miRNA to produce a miRNA precursor (pre-miRNA). The second cleavage takes place near the loop of the pre-miRNA to produce a miRNA/miRNA* duplex. The mature miRNA is then loaded into the RNA-induced silencing complex (RISC) and can guide the sequence-specific cleavage or translational inhibition of target mRNAs, as well as gene silencing through DNA methylation, whereas the non-incorporated miRNA* strand is usually degraded.
Through the use of next-generation sequencing, the small RNA component of the Arabidopsis and rice transcriptomes has been well characterized, more than in any other plant species (11). This is reflected in the miRBase database on the world wide web at .mirbase.org, release 16: September 2010), where 213 miRNAs are described for Arabidopsis whereas 462 miRNAs are described for rice. Besides rice, the identification of miRNAs through deep sequencing in other grasses including maize, wheat, and Brachypodium have been described (Wang et al., (2009) Plant Cell, 21:1053-1069; Wei B. et al., (2009) Funct Integr Genomics 9:499-511). The identification of rice, maize and wheat miRNAs from different tissues, developmental stages and stress-treatments, provides an opportunity to understand how miRNAs regulate the expression of genes influencing traits of agronomic importance.
High sucrose content is a highly desirable trait because sugar can be fermented to produce bioethanol as a source of renewable energy (3). Although sugarcane has been extensively used as a source of biofuel, its use as a model system to understand the genetics of carbohydrate metabolism is hampered by its complex genome, with several cultivars differing greatly in their ploidy levels (4). Sorghum instead, provides a better system to study the genetic basis of sugar accumulation.